1. Field of the Invention
The present invention relates to a coupler of a collapsible intermediate shaft or steering shaft of a steering device.
2. Related Background Art
A steering device of an automobile is constructed as shown in FIG. 5.
Numeral 1 denotes a steering wheel. The movement of the steering wheel 1 is transmitted to a steering gear 4 through a steering shaft (not shown) inserted into a steering column 2, and wheels are steered by the steering gear 4.
Since the steering shaft and an input shaft 5 of the steering gear 4 cannot usually be arranged on the same line, an intermediate shaft 3 is provided between the steering shaft and the input shaft 5, and the opposite ends of the intermediate shaft 3 and the end of the input shaft 5 are coupled by universal joints 6 so that the movement can be transmitted between the steering shaft and the input shaft 5 which are not on the same line.
In such a steering device of the automobile, in order to protect the driver in a collision, it is a usual practice to use a collapsible shaft which collapses lengthwise when an impact is applied thereto, as the steering shaft or intermediate shaft 3.
FIG. 6 shows a prior art construction which uses a collapsible shaft as the intermediate shaft 3.
The intermediate shaft 3 is constructed by coupling a shaft member 7 and a tube member 8 by synthetic resin 9.
A male serration 10 is formed on an outer periphery of an end of the shaft member 7, and a female serration 11 which is to engage with the male serration 10 is formed on an inner periphery of an end of the tube member 8. A pair of through-holes 12 are formed at the end of the tube member 8 at diametrically opposing positions, and a concave groove 13 is formed on the outer periphery of the end of the shaft member 7 around the entire circumference. The synthetic resin 9 filled in the groove 13 through one of the pair of through-holes 12 is driven into a small clearance between the male serration 10 and the female serration 11 to prevent play between the male serration 10 and the female serration 11 when the intermediate shaft 3 is rotated.
A yoke 14 of a universal joint is fixed to the end of the shaft member 7 by a bolt and a nut (not shown) and a yoke 15 of the universal joint is fixed to the end of the tube member 8 by welding.
As noted earlier, the synthetic resin 9 filled into the recessed groove 13 through the one through-hole 12 is driven into the fine clearance between the male serrration 10 and the female serration 11 to prevent play there between. However, if the precision of the components is not strictly defined, a sufficient amount of synthetic resin 9 may not be driven into the clearance between the serrations 10 and 11 due to the manufacturing tolerance of the components. In the prior art construction, the manufacturing cost of the collapsible shaft increases due to the precision of the components required to prevent the above problem.
Japanese Utility Model Laid-Open Application No. 63-104280 discloses a structure in which a concave portion is formed on the outer periphery of the male serration 10 so that a relatively large amount of synthetic resin is driven into the clearance between the concave portion and the female serration. However, the manufacturing cost of the collapsible shaft is still high because the machining of the concave portion takes a long time and the abrasion of a tool (end mill) to form the concave portion is rapid.
Japanese Utility Model Laid-Open Application No. 64-45576 discloses a structure in which a through-hole having the opposite ends thereof extended to the male serration is formed diametrically of the end of the shaft member, and the synthetic resin is filled in the through-hole. However, the manufacturing cost of the collapsible shaft is again high because the machining is troublesome and time-consuming and the abrasion of a tool (drill) for forming the through-hole is rapid.